I find the whole Wanda culpability thing as pointless as feat debates (spoiler: depends on the writer) but to me mutant does not mean it belongs to the X-Men. Mutants are part of the Marvel U. The erasure of her mutant status was done as a statement that mutants were essentially shoved in the corner. Restoring the twins to their mutant status would be a sign that Marvel is hopefully done with the segregation.